FIG. 2 is a schematic illustration of a well-known toner density detection apparatus for an electrophotographic apparatus. FIG. 3 shows the relationship between toner density and the permeability of a developing agent 6. The developing agent 6 is a mixture of toner 4 and carrier 5. FIG. 4 shows a conventional operational relationship between an electrophotographic apparatus and a rotary roll 3 of the toner density detection apparatus.
A toner density detection apparatus such as shown in FIG. 2 generally includes a permeability sensor 1, a special vessel 2 and a rotary roll 3. The toner density detection apparatus detects the toner density of a developing agent 6 consisting of a mixture of toner 4 and carrier 5 circulating inside the developer.
A portion of the developing agent 6 circulating inside the developer is fed into the special vessel 2, and the permeability of the developing agent 6 in the special vessel 2 is detected by the permeability sensor 1. The flow quantity and flow rate of the developing agent 6 in the special vessel 2 are kept constant by the rotary roll 3 provided at an outlet of the special vessel 2. Because the toner density and permeability of the developing agent 6 have a predetermined relationship as shown in FIG. 3, the toner density of the developing agent 6 can be determined by measuring the permeability with the permeability sensor 1. The operational relationship between the electrophotographic apparatus and the rotary roll 3 is shown in FIG. 4. As shown in FIG. 4, the electrophotographic apparatus and the rotary roll 3 are started at the same time and stopped at the same time.
In the conventional toner density detection method, when an instruction is given to stop operation of the electrophotographic apparatus, the rotating operation of the rotary roll 3 is stopped at the same time, as shown in FIG. 4. Thus, when the electrophotographic apparatus has been stopped, the special vessel 2 is full of developing agent 6. If the electrophotographic apparatus is not operated for a long period of time, the developing agent 6 inside the special vessel 2 can solidify due to the characteristics of the developing agent 6, the influence of the peripheral environment (high humidity), etc., so that it is difficult not only to exhaust the developing agent 6 out of the special vessel 2, but to supplement the developing agent circulating inside the developer into the special vessel 2 when the operation of the electrophotographic apparatus is restarted. As a result, it is impossible to detect the permeability, and it is likewise impossible to control the toner density. If the operation of the electrophotographic apparatus is continued in the above-mentioned state, various failures will occur in the quality of printing.